Physics final exam question: solidification age comparison

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA

Two samples currently have certain amounts of a radioactive isotope, embedded gaseous daughter element, and inert material (which is not involved in the decay process). Determine which sample has an older solidification age (as determined by radioactive dating), or if there is a tie. Explain your reasoning using properties of radioactive decay.

Solution and grading rubric:
  • p:
    Correct. The samples have the same solidification age, as they have the same ratio of radioactive isotopes today compared to when they first started, as the inert material does not factor into the decay process.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying properties of radioactive decay.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of radioactive decay.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 36 students
r: 0 students
t: 0 students
v: 2 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 9950):

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